Thermoelectric Properties And Phonon Manipulation Of Two-Dimensional Nanomaterials

Climate change is one of the most serious challenges facing humanity in the 21^stcentury.One of the main factors contributing to climate change is the increased consumption of fossil fuels.For energy and environmental security and future sustainable development,new clean and efficient energy technologies are urgently needed.Thermoelectric materials（TM）can convert waste heat into electricity,so TM play an important role in addressing the current global energy challenge of providing sustainable and clean energy.However,on the one hand,the energy conversion efficiency of the current TM is relatively low,and on the other hand,the current commercialized TM are mainly composed of heavy metal elements such as Bi,Te,and Pb.These elements are naturally low in abundance and the scarcity of original materials,leads to the cost of TM is too expensive.Therefore,it cannot be widely used so far.In this paper,we use non-equilibrium Green’s function,lattice dynamics,density functional theory（DFT）,Boltzmann transport and so on to evaluate the thermoelectric application potential of graphene-like（GR）,periodic porous graphene and two-dimensional monolayer Si₂BN,which are composed of resource-rich,low-cost and green light elements.As follows:（1）The graphene-like GR is a micro-bandgap semiconductor（～0.025 e V）.In order to widen its bandgap to improve the thermoelectric performance,we simulated tailoring the GR into GR nanoribbons.The results show that the band gap of GR nanoribbons is negatively correlated with the width of the nanoribbons,up to～1 e V.The thermoelectric properties of GR nanoribbons are also negatively correlated with width,with a maximum ZT value of about0.45 at room temperature.To further improve the thermoelectric properties of GR nanoribbons,we introduce a stub structure,which leads to a large reduction in phonon thermal conductance,which is due to the stub structure induced phonon local resonance in GR nanoribbon devices.Finally,the thermoelectric performance is significantly improved,reaching 1.8 at room temperature,and up to 3.4 at 800 K.（2）Periodically porous graphene NPG is an intrinsic semiconducting graphene with a band gap of～1 e V,synthesized in recent years.Therefore,there are great application potential in future nanodevices.The ultra-high thermal conductance of perfect graphene and the possible hindrance of porous to thermal conductance cast a mysterious veil on the thermal transport properties of NPG,which are crucial to its future applications.For this purpose,we tailor NPGs into nanoribbons to simulate nanodevices,and compare them with perfect graphene nanoribbons of the same size to study the thermal transport of NPG nanoribbons.The results show that the thermal transport of NPG nanoribbons with different chirality is obviously different.For NPG nanoribbons of the same width,the thermal conductance of the armchair type is about 2 times that of the zigzag type.And the thermal conductence of NPG nanoribbons is much lower than that of perfect graphene nanoribbons,and the thermal conductance of armchair-shaped and zigzag-shaped nanoribbons is only about 34%and 12%of that of graphene nanoribbons,respectively.Therefore,NPG has great application potential in the field of thermoelectricity.（3）The original 2D monolayer Si₂BN has metallic properties,and the almost negligible Seebeck coefficient brought by metallic properties is very unfavorable for thermoelectric transport.Therefore,in order to transformed Si₂BN into a semiconductor property,we adopt the method of surface functionalization,and use halogen atoms to adsorb on Si atoms on the surface of Si₂BN.So that the orbital hybridization type of Si atoms changes from sp²to sp³hybridization,and the originalπelectrons are by bind,thereby limiting the electron transport properties of Si₂BN to a certain extent to open the band gap.In addition,the electronic energy band,phonon thermal conductivity,and thermoelectric properties of Si₂BN-4X（X=F,Cl,Br,I）change monotonically with the increase of halogen atoms.The point is that the ZT value increases monotonically with the increase of the mass of the halogen atom,which is because the phonon thermal conductivity decreases monotonically with the increase of the mass of the halogen atom.Moreover,due to the band degeneracy effect,the ZT value of Si₂BN-4I is significantly improved,which is 16 times higher than that of pristine Si₂BN.